1. Field of the Invention
The present invention relates to an electron generating apparatus to be used for example in an ion source such as an electron beam excitation ion source.
2. Description of the Related Art
An ion source is generally used in an ion implantation apparatus for implanting ions as an impurity in, e.g., a semiconductor wafer and widely used in an apparatus for treating a surface of a semiconductor wafer or analyzing a solid body.
Conventional ion sources, e.g., a Freeman type ion source as follows, are widely used. A power is applied between a filament (cathode) and an anode electrode to generate a plasma from a predetermined gas, and desired ions are extracted from the plasma and used.
There is proposed in Published Unexamined Japanese Patent Application No. 62-278736 an electron beam excitation ion sources having a different construction from the above-mentioned Freeman type ion sources. A power is applied between a filament and an anode electrode to generate a first plasma from a predetermined gas, and electrons are extracted from the first plasma to be radiated onto a predetermined gas, thereby generating a second plasma. Desired ions are extracted from the second plasma and used.
According to the electron beam excitation ion source, a high ion current density can be obtained with a low ion energy, and the frequency of maintenance can be reduced to achieve high-efficiency processing due to long service life.
However, according to the ion source, the following problems are posed. The filament is sputtered at a unit (electron generating unit) for generating the first plasma by ions in the plasma, and the service life and maintenance of the electron generating unit are variously influenced.
For this reason, a countermeasure against the maintenance problems has been conventionally taken, e.g., only a filament body which is easily worn out by sputtering is arranged to be exchangeable. However, this countermeasure is not satisfactory.